Recently, research and development of a new generation of wireless communication systems after International Mobile Telecommunication 2000 (IMT-2000) is progressing. The 3rd Generation Partnership Project (3GPP) that determines international standardizations for mobile wireless communication systems is debating a new generation of wireless communication systems known as Long Term Evolution (LTE). One of its important objectives is to consider ways of improving characteristics of mobile station devices at cell/sector edges.
FIGS. 20A and 20B are plan views showing an example of a cell/sector configuration (see Non-Patent Document 1). As shown in FIG. 20A, one cell (e.g. cell c1) is adjacent to six cells (c2 to c7). A base station device (e.g. base station device 1) performs communication by dividing one cell (e.g. cell c1) into three sectors (sectors #11, #12, and #13). Cell c2 is divided into sectors #21, #22, and #23, cell c3 is divided into sectors #31, #32, and #33, cell c4 is divided into sectors #41, #42, and #43, cell c5 is divided into sectors #51, #52, and #53, cell c6 is divided into sectors #61, #62, and #63, and cell c7 is divided into sectors #71, #72, and #73.
FIG. 20B is an enlarged view of the region including the cell c1 and cell c2 of FIG. 20A. In FIG. 20B, an interface between two sectors belonging to a base station device 1 (here, the interface between sector #11 and sector #12, being a straight line connecting point P1 and point P2) is termed a sector edge. An interface between cells of two base station devices 1 (here, the interface between cell c1 and cell c2, being a straight line connecting point P3 and point P4) is termed a cell edge.
The base station device 1 employs a one-cell repetition system using an identical frequency where, to reduce affects of mutual interference between adjacent base station devices (cells), one-cell repetitions of frequency are implemented by, for example, multiplying the transmit signal with a different scrambling code for each cell, thereby smoothing the interference. The base station device 1 includes a plurality of sector antennas configured from directional antennas, a plurality of fan-like sectors (e.g. sectors #11, #12, and #13) are arranged in one cell (e.g. cell c1), and an identical frequency is repeatedly used. This cell/sector configuration enhances the frequency usage efficiency, and increases the subscriber capacity. However, cell edge/sector edge mobile station devices are at least affected by interference from other cells and other sectors, leading to a problem of deteriorating characteristics.
Macrodiversity is considered as one technique for solving this problem. Macrodiversity is a technique that uses a plurality of cells and sectors to make a diverse transmission of a transmit signal to a mobile station device.
FIG. 21 shows one example of a macrodiversity method (hereinafter macrodiversity method B) of transmitting from a plurality of sectors using signals created from data that differs between sectors. In FIG. 21, a base station device 1 transmits a signal S1 to sector #1 using an antenna a1, and transmits a signal S2 to sector #2 using an antenna a2.
Signal S1 is transmitted at time intervals t1b, t1d, and t1f, and is not transmitted at time intervals t1a, t1c, and t1e. In contrast, signal 82 is transmitted at time intervals t1a, t1c and t1e, and is not transmitted at time intervals t1b, t1d, and t1f.
For example, fast sector selection with transmission muting method can be used as the macrodiversity method B. In this method, while the base station device 2 transmits a signal after selecting a sector having high reception power or high received signal to interference plus noise ratio (SINR), and the selected sector is transmitting another sector stops transmission such as to suppress an interference component received by the transmitting sector (Non-Patent Document 2). There is a similar method where, instead of making another sector stop transmission as in fast sector selection with transmission muting, the transmission power is set to a smaller value than another sector, thereby relieving the interference component applied to the other sector.
FIG. 22 shows an example of a macrodiversity method (hereinafter macrodiversity method A) of transmitting from a plurality of sectors using a signal created from identical data between sectors. In FIG. 22, a base station device 1 transmits a signal S3 from sector #1 using antenna a1, and a signal S4 from sector #2 using antenna a2, to a mobile station device 2.
Signal S3 transmitted at time intervals t3a, to t3f, and signal S4 transmitted at time intervals t4a to t4f, are transmitted from the base station device 1 such that they become the same signal.
For example, soft combining method can be used as macrodiversity method A. This is a method where both sectors transmit identical signals created from identical data at identical timings to an identical mobile station device 2, increasing the signal component of the mobile station device 2 while suppressing the interference component (Non-Patent Document 3).
Since the macrodiversity methods A and B described above assume that synchrony among the sectors is obtained at the base station device 1, they do not require complex processes such as, for example, in a soft handoff between cells in a code division multiple access (CDMA) communication system shown in Patent Document 1, where a RAKE receiver performs synthesis by synchronizing signals from each cell.
Another example of a macrodiversity method of transmitting from a plurality of sectors using signals created from identical data between the sectors is space time transmit diversity (STTD), which transmits, from a plurality of sectors, signals that are encoded by space-time encoding between the sectors, and also obtains coded gain (see Patent Document 2). For example, a base station device space-time encodes a modulation signal with two STTD codes, transmits one STTD code from one sector, and simultaneously transmits the other STTD code from another sector. The mobile station device estimates the channel of each sector, and performs STTD decoding using the estimated channel of each sector Also, there is a method of obtaining transmit diversity using a plurality of transmission antennas in an identical sector, whereby closed loop transmit diversity (Non-Patent Document 4) where transmit weight is calculated based on channel information of the mobile station device 2 and the signal is transmitted after multiplying by the calculated weight with each antenna, is applied in transmit antennas of different sectors (hereinafter termed closed loop macrodiversity method), and such like. For example, a transmit weight that maximizes the reception power P of the mobile station device can be determined from equation (1) below.P=wHHHHw  (1)
Here, H represents the channel response for signals from each sector, and w represents the transmit weight of each sector; when using two sectors, H=[h1, h2], w=[w1, w2]T, where h1 is the channel response for a signal from sector #1, h2 is the channel response for a signal from sector #2, w1 is the transmit weight of sector #1, and w2 is the transmit weight of sector #2. The mobile station device feeds back the determined transmit weights to the base station device, which transmits the signals simultaneously using the fed-back transmit weights in each sector.
These macrodiversity methods A and B can enhance the received SINR of the mobile station device 2. Macrodiversity method B is realized by suppressing the interference component between sectors, while macrodiversity method A is realized by suppressing the interference component between sectors while increasing the signal component. Due to this increasing of the signal component by transmitting signals simultaneously from a plurality of sectors, macrodiversity method A can generally enhance the received SINR more than macrodiversity method B.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication, (JP-A) No. H0-172390    [Patent Document 2] Japanese Unexamined Patent Application, First Publication, (JP-A) No. 2003-23381    [Non-Patent Document 1] 3GPP TSG RAN WG1 Ad Hoc on LTE Sophia Antipolis, France, 20-21 Jun., 2005 R1-050587, “OFDM Radio Parameter Set in Evolved UTRA Downlink”    [Non-Patent Document 2] 3GPP TSG RAN WG1 Ad Hoc on LTE Sophia Antipolis, France, 20-21 Jun., 2005 R1-050624, “On Macro Diversity for E-UTRA”    [Non-Patent Document 3] 3GPP TSG RAN WG1 Ad Hoc on LTE Sophia Antipolis, France, 20-21 Jun., 2005 R1-050615, “Investigations on Inter-Sector Diversity in Evolved UTRA Downlink”    [Non-Patent Document 4] 3GPP TS 25.214, “Physical layer procedures (FDD)”.
However, conventional techniques consider only two sectors, and do not consider interference component applied to other sectors and other cells in a wireless communication system. This will be explained using soft-combining as macrodiversity method A, and fast sector selection with transmission muting as macrodiversity method B. While fast sector selection with transmission muting cannot obtain an additional increase in the signal component since it stops transmission from one sector, in comparison with when not using macrodiversity, it can suppress the interference component to other sectors adjacent to the transmitting sector, and the interference component to adjacent cells.
On the other hand, while soft-combining can additionally increase the signal component since it transmits identical data from both sectors at identical timings, in comparison with when not using macrodiversity, it cannot reduce the interference components to adjacent sectors and adjacent cells.
At the mobile station device 2, there are cases where deterioration in the received SINR is mainly caused by increase in the interference component, and it is possible to realize a requested transmission rate by using fast sector selection with transmission muting to suppress the interference component, and to apply a modulation scheme having the largest modulation value among applicable modulation schemes. On the other hand, at the mobile station device 2, there are cases where deterioration in the received SINR is caused both by deterioration in the signal component and increase in the interference component, and it is not possible to realize a requested transmission rate without using soft-combining to suppress the interference component while increasing the signal component. These states will be explained in detail with reference to FIG. 23.
FIG. 23 is a diagram of fluctuation in the received SINR of mobile station devices in sectors. The received SINR shown here represents an average value over time, rather than a momentary value that changes due to fading and the like. Mobile station device 2a is located in a middle region between two sector edges (here, a middle region between the sector edge of sectors #1 and #2 and the sector edge of sectors #1 and #3), at a point (area A) nearer to the base station device 1 than a middle point between the cell edge of the base station device 1 and the base station device 1; this mobile station device 2a has a large signal component from the base station device 1, low interference component from sector #2 and other cells, and high received SINR.
Mobile station device 2b is located in a middle region between two sector edges, at a middle point (area B) between the cell edge of the base station device 1 and the base station device 1, and, in comparison with mobile station device 2a in area A, has a reduced signal component from the base station device 1, and a deteriorated received SINR.
Mobile station device 2c is located in a middle region between two sector edges, at a point (area C) further from the base station device 1 than the middle point between the cell edge of the base station device 1 and the base station device 1, and, in comparison with mobile station device 2a in area A, has a reduced signal component from the base station device 1, increased interference component from other cells, and a greatly deteriorated received SINR.
Mobile station device 2d is located in a region between two sectors (here, the region of the sector edge of sector #1 and sector #2), at a point (area D) nearer to the base station device 1 than the middle point between the cell edge of the base station device 1 and the base station device 1, and, in comparison with mobile station device 2a in area A, has a greatly reduced interference component from another sector (here, sector #2), and a greatly deteriorated received SINR.
Mobile station device 2e is located in a region between two sectors, at a point (area E) further from the base station device 1 than the middle point between the cell edge of the base station device 1 and the base station device 1, and, in comparison with the mobile station device 2a in area A, has a greatly reduced signal component from the base station device 1, increased interference component from other cells and another sector (here, sector #2), and a deteriorated received SINR. That is, the mobile station device 2d in area D enhances its received SINR by reducing the interference component from other sectors. However, if the mobile station device 2e in area E merely reduces the interference component from other sectors, it obtains only a slight increase in the received SINR; it is therefore necessary to increase the signal component.